| Nationality | Name | Wins | Losses |
|---|---|---|---|
| Nationality | Name | Wins | Losses |
| Japanese | Ryunosuke Omori | 7 | 2 |
| Japanese | Kaito Ono | 51 | 6 |
| Rade Opačić | 17 | 6 | |
| Dutch | Henri van Opstal | 25 | 10 |
| Romanian | Ștefan Orza | 19 | 1 |
| Japanese | Kazuki Osaki | 40 | 5 |
| Japanese | Koki Osaki | 29 | 6 |
| Japanese | Fumiya Osawa | 29 | 19 |
| Japanese | Haruaki Otsuki | 43 | 14 |
| Dutch | L'houcine Ouzgni | 97 | 5 |
| Dutch | Alistair Overeem | 10 | 4 |
| Dutch | Valentijn Overeem | 1 | |
| Dutch | Jay Overmeer | 29 | 5 |
| Japanese | Keiji Ozaki | 26 | 18 |
| Japanese | Kaito Ozawa | 15 | 9 |
| Dutch & Turkish | Tayfun Ozcan | 86 | 10 |
| Turkish, Dutch | Serkan Ozcaglayan | 45 | 7 |
| Turkish | Yetkin Özkul | 68 | 16 |
| Serbian | Nenad Pagonis | 9 | 3 |
| Thailand | Pajonsuk | 127 | 33 |